This proposal by Dr. Shyu is a continuation of his work from the previous funding period. He has made significant progress and contributions to the understanding of ARE-mediated mRNA turnover and appears to have all the groundwork to continue to do so. The proposal investigates the role of hnRNP D (also known as AUF1) in the degradation of ARE-containing mRNAs and how post-translational modifications and associated proteins influence this activity. The specific aims below are designed to obtain answers to the following questions: 1. What are the roles played by stress-activated signaling pathways and the ubiquitin-proteasome pathway in ARE-directed mRNA turnover? 2. What is the ARE/protein decay complex necessary for ARE-dependent mRNA turnover? 3. What structural features of hnRNP D are necessary for its role as a destabilizing protein in ARE-mediated mRNA decay in vivo? 4. How does the ARE exert its destabilizing function? The investigator proposes to address this question by employing in vitro RNA decay systems to dissect and characterize the ARE-targeted mRNA decay.